Due to their high storage density, long data retention life, and relatively low cost, optical disks are becoming increasingly popular as a means to distribute information. Large format disks have been developed for storing full length motion pictures. The compact disk (CD), and more recent mini disk (MD) formats were developed and marketed for the distribution of musical recordings and have essentially replaced vinyl records. High-capacity, read-only data storage media, such as CD-ROM, have become prevalent in the personal computer field, while the new Digital Video Disk (DVD) format may soon replace videotape as the distribution medium of video information.
An optical disk is made of a transparent disk or substrate in which data, in the form of a serial bit-stream, is encoded as a series of pits in a reflective surface within the disk. The pits are arranged along a spiral or circular track. Data is read from the optical disk by focusing a low power laser beam onto a track on the disk and detecting the light reflected from the surface of the disk, which is modulated by the pattern of the pits on the disk. Optical and imaging systems detect the modulated, reflected, laser light and produce an electrical signal which may be decoded to recover the digital data stored on the optical disk. The recovered digital data, which may include error correcting codes and additional subcoded information, is further processed to recover the stored data, which may then be converted to audio signals or executable programs.
To be able to retrieve data from anywhere on a optical disk, the optical systems include a pickup assembly which may be positioned to read data from any disk track. Servo mechanisms are provided for focusing the optical system and for keeping the pickup assembly positioned over the track, despite disk warpage or eccentricity. Some optical disks systems rotate the disk to provide a constant linear velocity of the data track relative to the pickup assembly. Accordingly, the spindle motor rotates the disk at lower rotational speeds when reading the outer edge of the disk, and higher rotational speeds when reading data tracks near the center of the disk. Newer optical disk systems may use constant rotational speeds.
Because in most previously known optical disk systems the data is retrieved from the disk serially, i.e. one bit at a time, the maximum data transfer rate for an optical disk reader is determined by the rate at which the pits pass by the pickup assembly. The linear density of the bits and the track pitch is fixed by the specification of the particular optical disk format. For example, CD disks employ a track pitch of 1.6 .mu.m, while DVD employs a track pitch only about one-half as wide.
Previously known methods of increasing the data transfer rate of optical disk readers have focused on increasing the rate at which the pits pass by the pickup assembly by increasing the rotational speed of the disk itself. Currently, drives with rotational speeds of up to 12.times. standard speed are commercially available, and faster designs are on the horizon. However higher disk rotational speeds place increasing demands on the optical and mechanical subsystems within the optical disk player making such players more difficult and expensive to design and manufacture.
U.S. Pat. No. 5,426,623 describes a system to increase disk reading speeds by reading multiple tracks simultaneously. The data is read using a matrix detector that provides a track signal for each of the tracks being read. Two problems are encountered when attempting to read multiple adjacent tracks simultaneously. First, a single constant linear velocity cannot be maintained across all of the tracks being read, since for any given rotational speed, the inner tracks will have a lower linear velocity than the outer tracks; and second, a single data clock cannot be used to sample the data signals from all the tracks, since the phase of data signals from different tracks may not be in phase. Therefore, to read a large number of adjacent tracks synchronously, mechanisms must be provided to compensate for the difference in the linear velocities of the adjacent tracks and for the phase differences in the data signals.
Commonly assigned U.S. patent application Ser. No. 08/559,429, now U.S. Pat. No. 5,627,805, describes circuitry for synchronously reading multiple data tracks despite the difference in linear velocities of the multiple adjacent tracks. The present invention improves upon, and provides refinements to, that circuitry.
It would therefore be desirable to provide an optical disk reading apparatus and methods that provide high speed retrieval of information from an optical disk that permits multiple adjacent tracks of an optical disk to be synchronously read out.